I’m not going to tell you that you’re managing your information wrong. I would physically die if I had ever more than 20 tabs (my ADHD couldn’t handle it).
But I think you might be using the wrong tool. A browser (like Firefox) is not really designed as an information manager. It’s primary purpose is navigating and visualizing web pages. So when you talk about “a few megabytes of text and images” thats not what your browser sees. Your browser handles more than just the text and images. It also handles fetching and prefetching, a browser history for every tab, a JS context and much much more.
What you want is some kind of personalized archiving system that processes websites into machine processable (ie searchable) structures. Firefox is not that. Maybe data hoarder communities will have the answers you seek.
The toolset they use to run their containerlike system wrapping the games is called “pressure vessel”.